Fellowes was the go-to guy because of his previous successes writing about the British class system in movies such as "The Young Victoria," "Vanity Fair" and "Gosford Park (for which he won his Oscar).

His novels include upper-class skewering "Snobs" in 2004 and the debutante examination "Past Imperfect" in 2009. He's also acted in James Bond's "Tomorrow Never Dies" and television's "Monarch of the Glen." Fellowes made his film directorial debut with "Separate Lies" in 2005, which he once described as "a French film in English, about middle-class people being unhappily married."

So writing the book for "Mary Poppins" — which debuted in London in 2004 and hit Broadway in 2006 — and its veddy-propah-old-boy social stratification in Edwardian England was a natural for Fellowes.

"I loved the film," Fellowes said from his home Stafford House in Dorchester. "I think it is a lovely movie. Of course, in the movie Mary Poppins solves the problems of a family that didn't have any."

Fellowes gave Mary Poppins a problem … or two.

"People talk as if there are three classes when in fact there are 133," Fellowes said. "And everything we do is to nudge ourselves up and we worry if we've slipped downward a few levels."

That is the problem Fellowes gave George Banks, the stuffy father in the story. For the uninitiated, "Mary Poppins" was a series of novels by Australia-born author Pamela Lyndon Travers first published in 1934. The eight-book story centered on a mysterious nanny blown by the East wind to the Banks household at Number Seventeen Cherry Tree Lane in La Belle Epoque London to take care of the two unruly children, Jane and Michael (in the books there are five children by the end of the series).

In the books the father was mostly an absentee father. In the movie he was a rigid and unhappy man who grows emotionally. In the stage version Fellowes is given a cruel nanny back story.

"George is propped up by his insecurities," Fellowes added. "She [his wife Winifred Banks] knows it is a worthless situation. She knows his values are false.

It is very rare that we change the way people see us," Fellowes continued. "We are told that if we have this car or wear these clothes or live on that street we will be seen a certain way. People have always done it and people do it today. And then we transfer that anxiety onto others. That's what George does. He transfers his anxieties on to his wife and his children."

It is with the character of the wife Winifred Banks that Fellowes made his most dramatic changes.

"Of course P.L. Travers was obsessed with her father," Fellowes said. "If only he hadn't died and so on. So there is a sort of redemption of George Banks. But her relationship with her mother was different. So I think that there is a gap with Mrs. Banks. So we've had to re-invent Mrs. Banks. And we couldn't go with the suffragette [persona from the film version]; that doesn't chime with the zeitgeist today.

We needed an adult situation in the show. And that was that she used to be interesting. She was an actress. Now her husband and even her children are critical of her. I knew every woman in the audience would have some sort of connection to that. And not to sound too cliche, but that's what grounds the show and gives it a heart. And I think we also thought of an aunt of Richard Eyre[the director] who was an actress."

Another change from the silver screen version: "Mary Poppins" the stage musical has a villain.

"Ah yes, Miss Andrew," Fellowes said. "We needed a villain. Cameron Mackintosh insisted on it. What we really needed was the polar opposite of Mary Poppins. There's no villain in the movie. You need jeopardy."

And then, reflecting on the balancing act the musical's book has, he added: "A musical has to have something for every age. A musical is very expensive. It's a big investment. One needs to serve an adult audience as much as anyone else."

It seems to be working. Even though the show will close on Broadway this March after seven years, there have also been productions in the U.K., Sweden, Finland, Denmark, Hungary, Czech Republic, Estonia and Mexico. There are plans to take the show to France, China, Japan and Iceland.

On the need for nannies today, Fellowes offered: "I think we've reached a crisis of being too involved in their lives. We don't give our children enough space to grow up. Back then the nursery was a kind of kingdom for the children. They had a separate life from what went on downstairs. I think children rather liked it. But now one is a bad parent if you are not involved in every aspect of your child growing up. Of course nowadays you can't open the door and say, 'Go play.' That doesn't happen to any kid anymore. The streets just aren't safe."